Hydrocarbon drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information may comprise characteristics of the earth formations traversed by the borehole, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole is termed “logging.”
Drillers often log the borehole during the drilling process, thereby eliminating the necessity of removing or “tripping” the drilling assembly to insert a wireline logging tool to collect the data. Data collection during drilling also enables the driller to make accurate modifications or corrections as needed to steer the well or optimize drilling performance while minimizing down time. Designs for measuring conditions downhole including the movement and location of the drilling assembly contemporaneously with the drilling of the well have come to be known as “measurement-while-drilling” techniques, or “MWD”. Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as “logging while drilling” techniques, or “LWD”. While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For purpose of this disclosure, the term LWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
In LWD systems, sensors in the drill string measure the desired drilling parameters and formation characteristics. While drilling is in progress these sensors continuously or intermittently transmit the information to a surface detector by some form of telemetry. Most LWD systems use the drilling fluid (or mud) in the drill string as the information carrier, and are thus referred to as mud pulse telemetry systems. In positive-pulse systems, a valve or other form of flow restrictor creates pressure pulses in the fluid flow by adjusting the size of a constriction in the drill string. In negative-pulse systems, a valve creates pressure pulses by releasing fluid from the interior of the drill string to the annulus. In both system types, the pressure pulses propagate at the speed of sound through the drilling fluid to the surface, where they are detected various types of transducers.
Data transfer rates in mud pulse telemetry systems are relatively low, on the order of five bits per second or less of actual downhole data. Moreover, downhole devices that operate as negative-pulse systems draw power to operate the valve or valves that create the pressure pulses from a battery system with limited energy storage capacity. Thus, any method or system that either increases the effective data transfer rate, or provides for longer battery life (whether at existing data transfer rates or increased data transfer rates), would provide a competitive advantage in the marketplace.